Create 'mountComponentWithRender' for testing with slots

spec/javascripts/helpers/vue_mount_component_helper.js

+import Vue from 'vue';
+
 const mountComponent = (Component, props = {}, el = null) =>
   new Component({
     propsData: props,
@@ -25,4 +27,12 @@ export const mountComponentWithSlots = (Component, { props, slots }) => {
   return component.$mount();
 };
 
+/**
+ * Mount a component with the given render method.
+ *
+ * This helps with inserting slots that need to be compiled.
+ */
+export const mountComponentWithRender = (render, el = null) =>
+  mountComponent(Vue.extend({ render }), {}, el);
+
 export default mountComponent;